NOx removal rate of photocatalytic cementitious materials with TiO2 in wet condition

Building and Environment 112 (2017) 233e240

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NOx removal rate of photocatalytic cementitious materials with TiO2 in wet condition Dawa Seo, Tae Sup Yun* Department of Civil and Environmental Engineering, Yonsei University, 120-749, 50 Yonsei-ro, Seodaemun-gu, Seoul, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 September 2016 Received in revised form 16 November 2016 Accepted 22 November 2016 Available online 23 November 2016

Photocatalytic cementitious materials used in urban buildings are promising to mitigate air pollution sustainably. The changes of NO concentration in previous studies have been tested in dry condition while photocatalytic cementitious materials are mostly exposed to wetting and its water content continuously changes. This study presents the removal capacity of Nitrogen monoxide (NO) by using Titanium dioxide (TiO2) under wet condition. Under both dry and wet condition, NO removal rates were measured. For dry condition tests, dominant factors such as TiO2 particle size, TiO2 mass replacing ratio with respect to cement, and humidity were under consideration. The specimen under wet condition was subjected to the evaporation and the evolution of NO removal rate was continuously monitored. Results present that the recovery rate of removal capacity initially increased followed by the stationary phase where removal capacity stayed quasi-constant for a considerable time period in spite of continuous evaporation. Upon complete drying, the recovery rate was not fully restored due to the wetting experiment. The phenomena involved unique phases during evaporation were further discussed. The observation emphasizes that the consideration of wet condition is crucial to comprehend the NO removal capacity of photocatalytic cement-based materials in real urban environments. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Photocatalytic cementitious materials Nitrogen monoxide NO removal rate Titanium dioxide Evaporation

1. Introduction Enhancing total building performance has been a challenge to raise the energy efficiency and to reduce green-house gas in construction material fields. The growing urbanization and heavy traffic load cause the high population density and serious air pollution such that the National Ambient Air Quality Standards [1] for Nitrogen oxides (NOx) has been revised; 53 ppb in the annual mean value to 100 ppm in the hourly mean value [2]. This revision underscores that the exposure to short-term NOx concentration as even low level of NOx with 0.05e0.2 ppm results in the serious respiratory diseases such as an asthma and bronchitis in the elderly and the sick [3,4]. Since the road transport in urban areas discharges NOx which accounts for 42.3% of air pollutants and the near-roadway areas are exposed to 30e100% higher NOx than other sections, the near-roadway buildings in urban areas can be the optimal and an alternative means to reduce NOx for residents [5,6]. Titanium dioxide (TiO2), a photocatalytic material, has been

* Corresponding author. E-mail addresses: [email protected] (D. Seo), [email protected] (T.S. Yun). http://dx.doi.org/10.1016/j.buildenv.2016.11.037 0360-1323/© 2016 Elsevier Ltd. All rights reserved.

investigated for the purpose of air purification and known as an effective material by degrading harmful organic and inorganic particles from air pollutants such as SOx (sulfur oxide), VOCs (volatile organic compounds) and NOx [7]. The cement based architectural and pavement treatments often adopt TiO2 as an addictive elsewhere; roof top surface of Jubilee church in Rome, Italy, coating wall of Air France's headquarters in Paris, France, 280 m length of pavement in Chiba, Japan, 457.2 m length of Highway 141 at St. Louis, Missouri, U.S. [8,9]. The photocatalytic reaction of NOx reduction occurs with TiO2, prompted by UV-A region of the solar spectrum (l < 390 nm). Hydroxyl radicals produced by the activated TiO2, react with NOx and the innocent secondary products are made through the photocatalytic reaction of TiO2 [10e13]. The NO removal rate varies depending on environmental factors (e.g., temperature, pollutants emission, wind, humidity, abrasion by working load) and materials used (e.g., TiO2 mass replacing ratio, TiO2 size, water-cement ratio) in both laboratory and field scale experiments [14e18]. Those current studies have followed ISO standard [19] which outlines ‘dry condition’ of specimens [14,19e21]. However, the urban infrastructures and buildings in most urban provinces except the arid zone stay virtually under ‘wet

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condition’ because of rainfall and domestic sewage. Specifically, the average number of days under rainfall is 17.2 days in July from 2006 to 2016 in Korea, and St. Louis in U.S. records >100 days of rainfall per year [22,23]. In other words, the ISO standard becomes the maximum capacity of NO removal rate in 'dry condition' rather than practical working capacity in 'wet condition'. Therefore, as most urban structures experience the repeated wetting and evaporation, it is highly desired to assess the effect of water existence on removal capacity of photocatalytic cement-based materials. In particular, the evaporation occurring in the exposed surface causes the unremitting variation of removal capacity. Evaporation tends to follow two stages in general; 1) high drying rate and 2) falling drying rate [24,25]. At stage 1, the high evaporation rate for a short period is pronounced and controlled by vapor phase diffusion through the exposed surface. This phase takes place right after rainfall when the surface is completely wet. As the specimen becomes dry, the evaporation rate significantly decreases at stage 2, influenced by interconnected pore structures. The evaporation rate depends on the intrinsic characteristics of materials and external environmental conditions. Thus, evaporation rate in the field can vary. In laboratory test, while less porous concrete exhibits the fast drop of initial evaporation rate within 2 h, porous concrete shows the longer period of phase 1 and the average rate of phase 2 is higher than that of less porous concrete [26]. For instance, Nemirovsky et al. [27] reports that water in porous concrete with 60 mm in height can be evaporated in 3 days in August where the highest evaporation rate occurs. This study presents the experimental results of transient changes in NO removal rate of photocatalytic cementitious materials with TiO2 under wet condition. The fully wet photocatalytic cement-based specimen was subjected to natural evaporation with continuous monitoring of NOx concentration with time. The effects of TiO2 size, TiO2 concentration and humidity of inflow gas were further analyzed to comprehend the recovery behavior of NO removal rate of wet photocatalytic cementitious materials. 2. Materials and methods The experiment was conducted under two different schemes; 1) NO absorption and removal tests with varying TiO2 size, TiO2 mass replacing ratio with respect to cement and humidity under dry condition and 2) NO removal rate changes in partially wet and evaporative conditions under initially wet condition. Table 1 summarizes the test conditions in details. 2.1. Materials The mixed TiO2 comprising 85% of anatase and 15% of rutile with less than 35 nm (SG-TOP25, Sukgyung AT), commonly used in industries due to its high photoactivity was selected as a photocatalyst [3,28]. The large size of TiO2 with less than 100 nm and the same components (Sigma-Aldrich) was also tested to comprehend the size effect. The water-to-cement ratio was 1:2 and cement-to-sand weight ratio was 1:2. Cement and sand were mixed for 1 min, and the designated amount of TiO2 was added and Table 1 Test conditions for evaluating NO absorption and removal rate. Condition

Measured features

Controlling factors

Dry

NO removal rate NO absorption rate

-

Wet

Recovery rate of NO removal rate

TiO2 size [nm]: 35, 100 TiO2 mass replacing ratio [%]: 1, 3,5, 10 Humidity [%]: 5, 15, 25, 35, 50 Changes in the weight of water during evaporation

blended for another 1 min. Then, water was poured into the mixed powder and kneaded for next 2 min following the suggest procedures [29]. The mixture was cured within the water bath for 7 days and dried in the oven at 40
C for 24 h. The lateral surfaces of specimen were thorough wrapped to isolate the top surface as an activated section. Note that the properties of specimen are encapsulated in Table 2. The specimen size was 100 mm in width and 200 mm in length. The specimen height and corresponding test variables are summarized in Table 3. The thin layer with 10 mm in height (e.g., cement-based tile) was considered as the most optimal type with constantly substituted TiO2 by abrasion unlike TiO2 coated cementitious materials [20] and 60 mm in height was used to control the evaporation time for comparison. For dry condition test, TiO2 particles with 35 nm and 100 nm were applied in thin layer (10 mm in height) with 5% of TiO2 mass replacing ratio with respect to cement. For TiO2 mass replacing ratio test, TiO2 with 35 nm size particles replaced cement mortar with four mass ratios (1, 3, 5, and 10%). For wet condition test, specimens with 10 mm and 60 mm in height were made with 5% of TiO2 (35 nm in particle size) mass replacing ratio and were submerged within water-filled vacuum chamber under 104 torr for 48 h. 2.2. Methods The monitoring of NO concentration changes was specified in ISO 22197-1 [19] as illustrated in Fig. 1. The NO gas mixed with zero air, targeting 1 ppm of initial NO concentration, passed through the gas washing bottle for humidity control. The temperature and humidity were checked before the gas entered the closed reaction chamber, which was open-top featured with borosilicate glass allowing the UV light to penetrate without any interrupt (340 mm in length, 160 mm in width, and 90 mm in height). The light from UV lamp had 380 nm in wavelength with 10 W/m2 of the light strength. The slope between the inlet and specimen allowed NO gas to thoroughly flow through the specimen surface which was 5 mm apart from the glass. The entire system was covered with the blackout cloth to block other light sources. The concentrations of NO and NO2 were monitored every second by NOx analyzer (T200, TELEDYNE). 2.2.1. Dry condition test The specimen sat within the reaction chamber and 1 ppm of NO gas stably flowed under 50% of humidity. NO absorption was tested in the dark for 30 min without the UV light followed by NO removal test under UV light for another 30 min (Fig. 2a). Five different humidity values (5, 15, 25, 35, and 50%) of inflow humidity were set by Table 2 Properties of specimens. Cement mortar specimen Water- cement ratio Cement-sand ratio Curing condition Curing period

Titanium dioxide (TiO2)

1:2 Type Anatase (85%) þ Rutile (15%) 1:2 Water curing Application Mass replacement with 7 days respect to cement

Table 3 Detailed specification of tested specimens. Controlling factors

Height [mm] Size of TiO2 [nm] Mixing ratio of TiO2 [%]

TiO2 size TiO2 mass replacing ratio Humidity Change in the weight of water

10 10 10 10, 60

35, 100 35 35 35

5 1, 3, 5, 10 1, 3, 5, 10 5

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Fig. 1. Experimental configuration of NO concentration change tests.

passing through the gas washing bottle to figure out the effect of water molecular on the exposed TiO2. NO removal rate was monitored for 30 min for each case in the light. NO absorption rate (a) and NO removal rate (b) were defined as follows.

a¼

NOinitial  NOinactivated $100 ½% NOinitial

(1)

b¼

NOinitial  NOactivated $100 ½% NOinitial

(2)

Each test was repeated with three different specimens and the average values were obtained. 2.2.2. Wet condition test The changes in NO removal rate were evaluated for specimens with 10 mm (H10) and 60 mm (H60) in height during water evaporation. For one experiment of wet condition, a cycle of NO gas and zero-air repeated until the initially saturated specimen became dry. In one cycle, NO gas with 1 ppm flowed for 30 min and zero-air (0 ppm of NO) flowed for next 90 min to facilitate the evaporation

Fig. 2. Experimental schemes of (a) dry condition test and (b) wet condition test.

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Fig. 3. NO concentration change test result (NO absorption and removal) of H10 in dry condition (Humidity ¼ 50%, TiO2 mass replacing ratio ¼ 5%).

of water (Fig. 2b). Note that the NO removal rate was measured during NO flowing and the average of NO removal rate was obtained in one cycle. This cycle continued with the monitoring of NO removal rate and the changes in specimen weight. The wet condition was quantified as the normalized weight of water (un).

un ¼

wi wo

(3)

where wo is the initial weight of water in fully saturated specimen and wi is the weight of water in specimen during evaporation. As the NO removal rate in dry condition outlined the maximum capacity, the recovery rate (bn) was defined as follows.

bn ¼

bwet $100 ½% bdry

(4)

where bwet is the measured NO removal rate during evaporation and bdry is the NO removal rate in dry condition. Then, the calibrated recovery rate (b’n) can be calculated.



b0n ¼ bn þ bdry  bredry



(5)

where bre-dry is the NO removal rate of the dried specimen subjected to the repeated saturation. 3. Results 3.1. Dry condition test Fig. 3 shows the evolution of NO (red line) and NO2 (blue line) concentration of H10 specimen in dry condition. Before the specimen is put into the reaction chamber, the NO concentration remains the initial value of 1.007 ppm. The placement of the specimen in the closed reaction chamber causes the reduction of NO and NO2 by 0.030 ppm (2.09%) and 0.028 ppm (85.63%) denoted by A and B, respectively, in the darkness without the UV light. It is attributed to that the cement mortar itself tends to bind NOx molecules with the polarity [10,30]. As TiO2 becomes activated by UV light at t ¼ 45 min, the photocatalytic reaction causes the sudden drop of NO concentration to 0.545 ppm (C ¼ 45.87%) and the concentration of NO2 increases to 0.097 ppm as a by-product

Fig. 4. (a) NO absorption rate (a) and removal rate (b) for 35 nm and 100 nm TiO2 sizes with the TiO2 mass replacing ratio of 5%. (b) Evolution of NO absorption rate (a) and removal rate (b) with varying TiO2 mass replacing ratio (1, 3, 5, 10%). The size of TiO2 is 35 nm.

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(D ¼ 237.91%). The concentrations of NO and NO2 remain steady for 30 min without showing any capacity reduction in TiO2 reaction, and abruptly restore to the initial values of NO and NO2 by removing UV source. As the NO removal rate varies from 24 to 82% in available literatures (24.73% in Ref. [31]; 55% in Ref. [29]; 75% in Ref. [15]; 82% in Ref. [11]), the measured removal rate of 45.87% in Fig. 3 seems reasonably acceptable and representative. 3.1.1. NO absorption and removal rate Fig. 4a illustrates that the smaller TiO2 particle of 35 nm results in higher NO absorption rate (a) and removal rate (b) than 100 nm. As cement mortar absorbs NO, the intrinsic structures of specimens contribute to the difference between NO absorption rates depending on the TiO2 size. For the same mass replacing ratio (5%), the smaller TiO2 has the larger number of particles than TiO2 with 100 nm. As TiO2 accelerates carbonation process in cementitious materials, the larger number of TiO2 results in the large surface area and high porosity [32e35], to improve NO absorption activity. In addition, smaller TiO2 particle presents more available active surface on TiO2 particles [36]. Thus, TiO2 with 35 nm shows high NO removal activity with available active surface of TiO2 particles. NO absorption and removal rate in Fig. 4b tend to grow with increasing TiO2 mass replacing ratio with respect to cement up to 5%. It is due to the effect of the larger number of TiO2 particles in high concentration; enlarged surface area of cement-based specimen and absolutely high activated area of TiO2, as mentioned above [37,38]. However, both rates do not significantly increase beyond 5%, indicating that TiO2 particle more than 5% replacing ratio may not obtain sufficiently expanded surface areas. Also, as the excessive TiO2 leads to the faster recombination of electronhole which is vital in photocatalytic reaction, so that photocatalytic reaction partially becomes inactive [11,32,35,39]. Therefore, 5% of TiO2 mass replacing ratio seems to be most efficient considering economic mixture ratio, workability so as to achieve the maximum NO removal rate by photocatalytic reaction.

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decreases at relatively high humidity and low contamination level [41]. In other words, the photocatalytic reaction is retarded because of the reduced activation site of TiO2 to the pollutants. However, as the dynamic equilibrium for occupying active sites between water molecules and pollutants is achieved by further competition [41], the retarded photocatalytic reaction is able to keep a range of constant photocatalytic capacity during the test.

3.2. Wet condition test Fig. 6 shows the changes in the normalized weight of water (un) and recovery rate (bn) for specimens with 10 mm (H10) and 60 mm (H60) in height. Each point indicates the measured data obtained from a single cycle shown in Fig. 2b. For H10 specimen (Fig. 6a), un gradually decreases with time and the corresponding bn sharply increases at the early stage of evaporation. There exists a stationary period where the recovery rate remains quasi-constant with 60e70% level, denoted by shadow (region A) despite of continuous decrease of un. The recovery rate is restored up to 96.5% of NO removal rate with respect to dry condition while un still remains 0.3. For H60 specimen, bn also rapidly increases at the initial period of evaporation and the stationary period, region C, appeared. In terms of developing pattern, the overall tendency bn of H60 looks similar with H10 and the changes in un during stationary periods

3.1.2. Humidity Fig. 5 shows that NO removal rate (b) linearly decreases with increasing humidity of inflow; 10% increase in humidity causes the average 7% of removal rate reduction. Note that the references were added on the results to help understanding [10,53]. Excessive water molecules in high humidity compete with the pollutant molecules to occupy the active sites of TiO2 [15,40]. Then, oxidation rate

Fig. 5. Effect of humidity on NO removal rate (b) for the specimen (TiO2 mass replacing ratio ¼ 5%, size of TiO2 ¼ 35 nm).

Fig. 6. Normalized weight of water (un) and recovery rate of NO removal rate (bn) for specimens with (a) 10 mm (H10) and (b) 60 mm (H60) in height.

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for both H10 and H60 specimens are analogous (e.g., 0.75 to 0.4 noted by region B and D). Note that H60 specimen was oven-dried after 330 h of natural evaporation due to slow evaporation rate. However, the stationary period seems to be more stably developed with longer period (region C) than the period of H10 (region A). Also, the stationary period of H60 specimen, region C, exhibits the lower recovery rate of 30e40% than region A, 60e70%. In addition, the final recovery rate of H60 was 72% of initial removal rate at dry condition. To identify the exact recovery capacity of H60 after wetting experience, NO removal rate (b) was measured again for oven-dried H60 specimen. Upon the completion of test, NO removal rate (b) was measured again for oven-dried H60 specimen. Results present that b is decreased to 72% of that in dry condition after wet condition test. As the abrasion rarely affects the capacity of TiO2 mixtures [42], the specimen structure would be solely modified during photocatalytic reaction. Its consequence can be explained as follows; when aged cement paste experiences rewetting after curing, the crystallization (i.e., hydration, carbonation) continues and the byproduct fills pore space. Considering the temperature range of tested specimens, carbonation process takes place by changing the internal structure of cement-based materials in wet condition, especially for young and poorly-cured cement [43e46]. The produced carbonate increases the density and decreases the pore size and permeability

[47e50]. As even micro structural change in pores and surface layer influences the decrease in NO removal rate [51,52], carbonation would cause the decrease in NO removal rate of H60 specimen after the wet condition test. Similarly, Diamanti et al. [52] observed that carbonation results in the degradation of self-cleaning of TiO2, which is one of representative characteristics of TiO2. The direct comparison of H10 with H60 specimen is feasible by plotting the calibrated recovery rate (b’n) with the normalized weight of water in Fig. 7. An additional NO removal rate test was performed with another H60 specimen (H60-1). The recovery rate of NO removal rate is inversely proportional to the normalized weight of water. As evaporation normally begins from the surface, the drying front gradually advances into the specimen. Then, TiO2 particles become exposed and activated wherever the drying front passes. However, crystallization in cement-based materials occurs continuously and the water evaporation elevates humidity in closed chamber to take a toll on recovery as explained above until the specimen remains in wet condition. Based on those effects, the evolution of recovery rate is divided into three phases; rapid recovery phase I, stationary phase II, and final recovery phase III. Phase I (rapid recovery): The NO removal rate is restored proportionally as un decreases from 1 to 0.7. The saturated condition initially hinders the direct exposure of TiO2 to the pollutants, and the evaporation facilitates the TiO2 activation.

Fig. 7. Calibrated recovery rate of NO removal rate (b'n) with normalized weight of water (un) for H10, H60, and H60-1 specimens.

Fig. 8. Schematic illustration of photocatalytic cement-based materials in wet condition.

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Phase II (stationary reaction): un keeps decreasing from 0.7 to 0.4 whereas b’n remains at 60e70%. The continuous evaporation results in the increase of humidity and rewetting of TiO2 by vaporized water molecules from pores. The water molecules existing near TiO2 particles make the photocatalytic reaction less stimulated as illustrated in Fig. 8. This negative effect counteracts the positive effect of increased uncovered TiO2 particles by evaporation. This balance is represented by the quasi-constant recovery rate. Phase III (final recovery): As the evaporation front becomes deep into the specimen, its rate decreases and the TiO2 particles possibly activated by UV light are mostly exposed. Then, b’n begins increasing again toward the maximum removal capacity. This observation underscores that the NO removal rate designated in the engineering standard outlines the maximum capacity without reflecting the wetting condition commonly occurring in urban areas such that the wetting-dependent removal capacity should be under consideration. From the suggested importance of wet condition, the future experiments with better quality of specimen and repeated tests with various materials would be expected to provide more practical application of the test results.

[12]

4. Conclusion

[13]

This study reports the experimental results of NO removal rates for photocatalytic cementitious materials under dry and wet conditions to emphasize the significance of wetting condition in conjunction with dominant factors. The following conclusions are made;  TiO2 particles with smaller size show the higher NO absorption rate and removal rate than the larger one in dry condition. Both rates increase with growing TiO2 mass replacing ratio with respect to cement up to 5%. The increase in humidity causes the decrease in NO removal rate.  Under wet condition, the recovery rate dynamically changes during evaporation and three unique phases of rapid recovery, stationary reaction, and final recovery. In particular, the calibrated recovery rate stays at 0.6e0.7 despite the continuous evaporation, regardless of the specimen height. The excess water molecules in high humidity hinder TiO2 activation while the drying front continuously advances during evaporation. These effects are counteracting to make the stationary phase.  Rewetting photocatalytic cementitious material lowers down NO removal rate. Observations suggest that the wetting directly affects NO removal rate of photocatalytic cement-based materials, and thus, wet condition should be considered as a vital impact factor to understand the practical sustainable capability of photocatalytic cement-based materials against air pollution. Acknowledgements This work was supported by the Korea CCS R&D Center (KCRC) grant and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2012-0008929, 20110030040, No. 2016R1A2B4011292). References [1] National Ambient Air Quality Standards (NAAQS), https://www.epa.gov/ criteria-air-pollutants/naaqs-table, U.S. 2010. [2] Environmental Protection Agency (EPA), Review of the Primary National Ambient Air Quality Standards for Nitrogen Dioxide: Risk and Exposure Assessment Planning Document, 2015. North Carolina, U.S. [3] J.J. Kim, S. Smorodinsky, M. Lipsett, B.C. Singer, A.T. Hodgson, B. Ostro, Traffic-

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